The present invention relates to a method for recovering etchant from etching waste liquid containing iron chloride, and more particularly a method for recovering etchant and other usable material from etching waste liquid containing iron chloride which is produced at the time of etching lead frames for IC (Integrated Circuit) or LSI (Large Scale Integrated circuit) or shadow mask of Brown tubes with etchant wherein metal ions such as copper ion and nickel ion having a lesser ionization tendency than iron ion are removed from the etching waste liquid.
Conventionally, lead frames for IC or LS1 or shadow masks of Brown tubes are produced by eroding portions of plate-like members made of a material such as copper or ferro-nickel alloy with etchant which contains a considerable amount of ferric chloride.
With such an etching processing, the ferric chloride is reduced to ferrous chloride so that the concentration of ferric chloride in the etchant is gradually lowered thus the etching efficiency of the etchant is also decreased. Accordingly, the etchant must be periodically replaced with fresh etchant which contains a sufficient ferric chloride while the used etchant having poor ferric chloride concentration is discharged from the system as etching waste liquid. The etching waste liquid, however, contains a considerable amount of usable metal ions such as copper ion or nickel ion besides iron ion after the etching processing. Furthermore, if these metal ions could be effectively removed from the etching waste liquid, reusable etchant could be obtained.
In view of the above, Japanese laid-open patent publication HEI 1-167235 proposes a method for recovering etchant and usable materials from etching waste liquid containing iron chloride. In this method, iron scrap or iron powder is mixed into etching waste liquid to reduce the metal ions such as copper ion and nickel ion having lesser ionization tendency than iron ion to recover usable metals such as copper and nickel and the liquid from which the usable metals are removed is utilized as etchant. In this method, however, the grade of the collected copper and nickel is low, while the use of iron scrap considerably lowers the speed of reaction between the etching waste liquid and the iron scrap.
To overcome such drawbacks, Japanese laid-open patent publication HEI 6-127946 proposes another method for processing etching waste liquid containing iron chloride. In this method, iron powder is mixed into strong acid waste liquid containing nickel and a small amount of chromium in the ferric chloride aqueous solution so as to control the oxidation-reduction potential (ORP) and iron ion concentration. Accordingly, copper and nickel ion eluted or dissolved in the strong acid waste liquid are precipitated in order as the result of electrolysis and foreign materials such as ferric hydroxide are removed thus producing etchant.
In the above Japanese laid-open patent publication HEI 6-127946, however, since the removal of metal ion in the etching waste liquid is carried out by mixing the iron powder with the etching waste liquid in the mixing vessel making use of an stirring blade driven by a power-operated motor, the method still has following drawbacks 1 to 5.
1 It is a prerequisite for the effective processing of the etching waste liquid to hold a considerable amount of iron powder in the etching waste liquid contained in the mixing vessel and to keep such iron powder in a favorable dispersed and floating condition in the etching waste liquid. However, the mixing vessel which produces a chemical reaction by the mechanical stirring blade suffers from the increase of processing capacity in terms of restriction on facility (e.g. a power-operated motor capable of agitating a large-sized stirring blade being extremely expensive) so that the method is not suitable for processing of a large amount of etching waste liquid.
2 To uniformly disperse and suspend a considerable amount of iron powder having a high specific gravity in the etching waste liquid, power necessary for driving the stirring blade becomes large pushing up the running cost of the mixing vessel.
3 Since iron powder is stirred at a high speed by the mixing blade, iron powder comes into contact with the inner wall of the mixing vessel and the outer surface of the mixing blade. Accordingly, the wear rate of the mixing vessel and the stirring blade is accelerated resulting in increase of expense necessary for the maintenance of the mixing vessel.
4 Since iron powder tends to settle at corners of the mixing vessel thus producing dead spaces where the mixing of iron powder and etching waste liquid are not carried out sufficiently, the overall mixing efficiency is low. Accordingly, the mixing vessel necessitates a considerable processing time for reducing metal ion such as copper ion and nickel ion having lesser ionization tendency than iron ion in the etching waste liquid.
5 Although the iron powder having a small particle size is favorably suspended in the etching waste liquid, the use of iron powder having such a small particle size pushes up the running cost.
In an operation to reduce the foreign metals making use of the reaction between the etching waste liquid and the iron powder and thereafter remove the reduced foreign metals, in case the ferric chloride is still contained in the etching waste liquid, the iron powder is first consumed by the reaction between the iron powder and the ferric chloride in the mixing vessel so that the total amount of ferrous ion and ferric ion in the final etching waste liquid is increased. To regulate the concentration of iron ion in the etching waste liquid at a desired or predetermined level, the final etching waste liquid must be diluted by water. Such a dilution, however, produces an excessively large amount of etchant. In this manner, when the collected etching waste liquid is processed using the iron powder to recover the usable etchant, the amount of etchant after processing exceeds the amount of etching waste liquid before processing and there arises a problem on handling of such an excess of etchant.
Meanwhile, Japanese laid-open patent publication SHO 57-44724 proposes a method for producing iron powder which can be preferably used for the above-mentioned etchant and metal recovering method. This iron producing method substantially comprises following four processes. In the first process, dust which is produced in a converter which blows oxygen for refining of steel is collected in a wet dust collecting method and the dust is classified such that the classified dust includes equal to or less than 30% of the dust particles having a particle size less than 44 .mu.m. In the second process, foreign materials such as scale and slag are peeled off from the dust particles having a large particle size by a wet-type finely crushing device. In the third process, peeled-off foreign materials are removed from the dust particles so as to produce metal iron. In the fourth process, the metal iron is refined to produce iron powder which can be preferably utilized in the above-mentioned etchant and metal recovering method.
However, in this method, since the process for producing iron powder by removing foreign materials from the dust is carried out by classification, thin layer sieve separation or magnetic separation, the process suffers from a low efficiency. Therefore, the production of the iron powder on a mass production basis is difficult. Furthermore, since the fourth process for refining metal iron is carried out by the combination of refining of iron oxide and the infiltration of dilute acid, the production of iron powder becomes costly.